Input modules and in particular input modules for use in sensing the condition of external devices are well-known in the art. Traditional input modules are used to sense the presence or absence of an electrical signal above or below a threshold level by use of impedance dividers, Z, to define current levels, including minimum and maximum load currents as well as threshold voltage levels, including minimum and maximum trip voltages. The typical characteristics of such prior art input modules are defined approximately by the equation: EQU V=Z*I, (1)
where Z=Z.sub.in +Z.sub.threshold. Such a relationship is by definition a linear function of voltage or current. Two standards have been defined by the International Electronic Committee (IEC) with respect to active or power available thresholds. Table 1 describes the basic characteristics of both the IEC type 1 and the IEC type 2 operating voltages as a function of operating current for DC, 120 VAC and 240 VAC input signals.
TABLE 1 __________________________________________________________________________ IEC TYPE 1 IEC TYPE 2 DC 120 VAC 240 VAC DC 120 VAC 240 VAC __________________________________________________________________________ OFF VOLTAGE -3 TO 5 0 TO 20 0 TO 40 -3 to 5 0 to 20 0 to 40 LEVEL VDC VAC VAC VDC VAC VAC OFF CURRENT 0 TO 15 0 TO 15 0 TO 15 0 to 30 0 to 30 0 to 30 LEVELS ma ma ma ma ma ma ON VOLTAGE 15 to 30 79 to (1.1*120) 164 to (1.1*240) 11 to 30 74 to (1.1*120) 159 to (1.1*240) LEVELS VDC VAC VAC VDC VAC VAC ON CURRENT 2 to 15 2 to 15 3 to 15 6 to 30 6 to 30 7 to 30 LEVELS ma ma ma ma ma ma __________________________________________________________________________
The type 1 inputs are typically used in industrial environments. This input standard absorbs less power per sense point or input than a type 2 standard, thereby allowing a relatively large number of points to be contained on a single printed circuit board. An example of this type of input is an input module which has a minimum ON threshold current of two milliamps at 15 VDC in a circuit which has linear current curve to V.sub.max (30 VDC), so that at V.sub.max current equals 4 milliamps. The initial power absorbed by the circuit is therefore 30 milliwatts (P=V*I) at the ON threshold point and reaches 120 milliwatts at V.sub.max. The total power in a thirty-two input module printed circuit board is therefore 3.84 watts.
The ratio between threshold power and maximum power is described by equation (2): EQU P.sub.max =k*P.sub.threshold, (2)
where EQU k=(V.sub.max /V.sub.threshold).sup.2. (3)
If equation 3 is applied to a type 2 input, the minimum results are: EQU k=(30/11).sup.2 =(2.7272).sup.2 =7.438. (4) EQU P.sub.threshold =V.sub.threshold *I.sub.threshold =11*6*E-3=0.066 watt.(5) EQU P.sub.max (assuming a 6 milliamp ON threshold current)=k*P.sub.threshold =7.438*0.066=0.4909 watt/per input. (6) EQU P.sub.total for 32 input points=32*0.4909=15.71 watts. (7)
As seen in FIG. 1, prior art linear circuits designed to meet the type 2 minimum ON current (that is 6 milliamps at 11 volts), absorb greater than 13 milliamps at 24 volts and greater than 16 milliamps at V.sub.max 30 volts. The improved DC input module according to the present invention provides for a high density input module which consumes smaller mounts of power per input point by modifying the current consumed for each input point after the ON state threshold voltage and current are exceeded.